This invention relates to a brake-force distributor for dual-circuit brake systems which is adapted to influence a connection of the first brake circuit between the brake-pressure source and wheel-brake cylinders, for which purpose it has a piston adapted to be acted upon in the closing direction by the pressure of the first brake circuit in opposition to a control force, the piston having a valve member adapted to close the connection at a valve seat, and which has an arrangement preventing closing of the connection in the event of failure of the second brake circuit.
Such a brake-force distributor is known from the German published patent application Dt-AS No. 1,961,941. In this application, the piston has a coaxial extension slidably sealed in the housing and acted upon by the pressure in the second brake circuit against the control force. The valve member is formed directly at the piston in its middle area and cooperates with a ring seal which surrounds the piston and is slidable in the housing and forms the valve seat. In the closing position, the piston has in its middle area a surface acted upon by the pressure of the brake-pressure source of the first brake circuit in the direction of the control force, and another surface acted upon in opposition to the control force by the pressure supplied to the wheel brake cylinders connected to the first brake circuit. In this method, all the surfaces are proportioned in such a manner that the piston is allowed to reach and remain in the closing position only if the extension is acted upon by the pressure of the second brake circuit. In the event of failure of the second brake circuit, it is always the forces acting in the direction of the control force that predominate. It is a disadvantage in this method that during normal operation the pressure of the second brake circuit has immediate influence on the piston and its movement and thus on the controlled pressure of the first brake circuit. As a result, any differential pressure between the two brake circuits caused by the brake-pressure source affects the controlled pressure of the first brake circuit. It is a further disadvantage that, due to the extension, the diameter of the valve seat has to be relatively large to ensure that suitable surfaces can be formed in the middle area of the piston.
From the German Pat. No. 1,655,294 a brake-force distributor of the type initially referred to is already known in which the second brake circuit has no immediate influence on the piston. This patent provides a retaining spring adapted to act upon the piston additionally in the direction of the control force. During normal operation, the retaining spring is spaced apart from the piston through an annular piston which is acted upon by the pressure of the second brake circuit. Thus, the retaining spring is effective only when the second brake circuit has failed. It is obvious that such a retaining spring is on principle a disadvantage because it has to be pre-loaded to a very high degree to be able to keep the piston in the open position, and in addition, during normal operation, it also has to be kept spaced apart from the piston against this preload by means of the annular piston which is acted upon by the pressure of the second brake circuit. This method draws a substantial amount of pressure fluid from the second brake circuit. Furthermore, such a brake-force distributor has high production requirements and necessitates very large dimensions.